A droplet actuator typically includes one or more substrates configured to form a surface or gap for conducting droplet operations. The one or more substrates establish a droplet operations surface or gap for conducting droplet operations and may also include electrodes arranged to conduct the droplet operations. The droplet operations substrate or the gap between the substrates may be coated or filled with a filler fluid that is immiscible with the liquid that forms the droplets. Droplets that are processed in a droplet actuator can have any concentration of a substance-of-interest, such as genetic material. This can be problematic, especially when mixing and/or processing droplets of different concentrations. Therefore, new approaches are needed for managing the concentration of droplets on a droplet actuator.